The present invention relates to flash memory devices and, more particularly, to a method for programming a flash memory device, which prevents interference between memory cells when the memory cells are programmed.
A memory cell array of a NAND flash memory device includes a plurality of memory cell strings connected to bit lines. Each of the memory cell strings includes a drain select transistor, a plurality of memory cells and a source select transistor. A gate terminal of each of the memory cells is connected to a word line, and operates in response to a signal received from the bit line and the word line.
FIG. 1 illustrates a conventional method for programming a flash memory device. The flash memory device includes memory cell string having seven memory cells. The method for programming the flash memory device is performed such that electrons move to a floating gate in a channel of a memory cell. The memory cell may be selected by Fowler Nordheim (FN) tunneling.
A voltage applied to word lines WL0 to WL6 while programming the memory cell string is at least 15 V. A selected memory cell string in which the selected memory cell is included is maintained at 0 V. The 0 V at the selected memory cell string increases the potential difference between the word lines and the channel by a maximum amount, thereby facilitating FN tunneling.
When a memory cell string has not been programmed, (i.e., an unselected memory cell string) electrons are not moved by FN tunneling. Thus, the potential difference between the word lines and the channel is minimized. When memory cell string is programmed, a negative potential exists at the floating gate even though a high word line voltage is applied. Accordingly, the potential at the channel is significantly decreased and lowers a channel boosting potential level.
A conventional method of maintaining the channel at a high boosting potential level applies a high gate voltage to all unselected word lines. However, the load of a flash memory device maintains a high word line voltage, and a pass disturb phenomenon (i.e., an incomplete program phenomenon) occurs at unselected memory cell strings resulting in unreliable memory devices.